1. Field of the Invention
The present invention relates to the field of microrelays.
2. Prior Art
Microrelays are currently being developed for low frequency and RF switching applications. A class of these devices is operated by electrostatic force and provides low form factor, low power consumption and excellent signal isolation capabilities. In general, electrostatic microrelays consist of four electrodes and an actuator (four terminal devices). Two electrodes, called the actuation electrodes, provide the attractive force for the actuator on application of an electric potential (voltage) difference between an electrode on the actuator and a fixed actuation electrode. The other two electrodes, called contact electrodes, switch the signal of interest when contacted and shorted together by an otherwise isolated, conductive area on the actuator. Such electrostatically operated microrelays have great potential in various markets, including automatic test equipment and telecommunications markets.
Typically in a microrelay, the contacts have to be at least 10 microns apart in the relay switch open condition to achieve good electrical breakdown and isolation performance. One known fabrication technique involves forming the actuator on a substrate, the actuator being separated from the substrate by a sacrificial layer that is etched away near the end of the fabrication process. However, increasing the gap between the actuator switching electrode and the fixed switching electrodes requires very thick sacrificial layers during the fabrication process, which is a non-trivial operation. Other schemes such as forming a wedge actuator with a controlled bending of the released actuator by built in stress layers is also difficult to control.
In addition, electrostatically operated microrelays can exhibit erratic operating characteristics if not suitably energized. In particular, the actuator electrodes providing the electrostatic operating force due to the voltage difference between the electrodes should not touch, as touching will short out the voltage difference, potentially damaging the relay and at best, temporarily removing the electrostatic actuating force. One way to avoid this is to put a layer of insulation on one or both actuating electrodes. However electric charge can build up on the insulating layers, providing a substantial electrostatic force on the actuator when the actuating electrodes are at the same voltage, or detracting from the electrostatic force on the actuator when the actuating electrodes are at intended actuating voltage differences. This effect can be minimized by grounding one electrode and driving the other electrode with a zero average voltage square wave, or driving the two actuating electrodes with complementary zero average voltage square waves. However, because the electrostatic force obtained is proportional to the square of the voltage difference between the actuating electrodes, the electrostatic force, when present, is always attractive. There is no repelling force that may be generated to open and hold the microrelay relay contacts open.